1. Field of the Invention
The present invention relates to a liquid crystal display device having a circuit composed of a thin film transistor, which may be referred to as a TFT hereinafter, and a method of driving the same. The present invention relates to, for example, an electro-optical device, a typical example of which is a liquid crystal display panel, and an electronic instrument on which such an electro-optical device is mounted as a component.
2. Description of Related Art
In recent years, attention has been paid to the technique of using a semiconductor thin film (thickness: about several nanometers to several hundreds of nanometers) formed on a substrate having an insulated surface to make thin film transistors (TFTs). A thin film transistor is widely applied to electronic devices such as an IC and an electro-optical device. It is particularly desired to develop the thin film transistor as a switch element of a liquid crystal display device.
In an active matrix type liquid crystal display device, its pixel portion is composed of several tens of pixels to millions of pixels which are arranged in a matrix form. Pixel TFTs are arranged in the respective pixels, and charges which go into and out from pixel electrodes connected to the respective pixel TFTs are controlled by the switch function of the pixel TFTs.
Recently, an active matrix type liquid crystal display device has been spreading not only as a display of a notebook-size personal computer, which has been widely known up to now, but also as a display of a desktop personal computer.
In a personal computer, it is desired to display plural pieces of information (including character information and image information) at a time. Thus, the display ability of the personal computer has been improved. That is, the resolution of images has been made high and the gradation number of display has been made large. (It is also desired to attain full-color display.)
With such an improvement in the display ability of a personal computer, an improvement in an active matrix type liquid crystal display device as the display device thereof has been advanced. Thus, in recent years, attention has been paid to an active matrix type liquid crystal display device in a digital driving mode which is easily interfaced with a personal computer, makes high-speed driving of its driver possible, and can realize an improvement in the display ability thereof.
In recent years, it has been increasingly desired to make the gradation number of an active matrix type liquid crystal display device larger. As one of the means for this, there has been developed an active matrix type liquid crystal display device in a digital driving mode having a driving circuit of a digital-input/analog-output type.
Into an active matrix type liquid crystal display device in a digital driving mode, digital video data are inputted from a data source of a personal computer or the like. An active matrix type liquid crystal display device having a digital driver needs a D/A converter circuit, which may be referred to as a digital-analog converter (DAC), for converting digital video data inputted from the outside to analog data (gradation voltages). The D/A converter circuit can be classified into various types.
One of the characteristics of an active matrix type liquid crystal display device having a digital driver is that pixels corresponding to one line can be simultaneously driven, that is, the so-called line-successive driving can be relatively easily realized.
In an analog driving mode, limitless gradation can be displayed. In a digital driving mode, however, the gradation (or the gradation number) which a display device realizes corresponds to a bit number. The gradation (or the gradation number) is the number of steps of brightness which can be represented.
The following will describe a conventional driving method. FIG. 12 is a driving timing chart of a conventional liquid crystal display device.
Display of one screen is referred to as “one frame (Tf)”, and a time necessary for displaying the one frame is referred to as “one frame period” hereinafter.
First, display in the first frame period will be described. About the first frame, a picture signal D1 is supplied to the corresponding pixel TFT so as to display an image.
About display in the next frame period, a picture signal D2 is supplied to the corresponding pixel TFT in the same way as in the first frame period, so as to display an image.
Thereafter, display of continuous frames is performed in the same way, so as to form a picture. Hitherto, 60 frames have been displayed per second. Therefore, the time for writing picture signals in the pixels is long so that flicker is generated to be conspicuous.
When gradation display is performed, there arises a problem that afterimages are liable to stand out because of a long response time of liquid crystal. Particularly in the case that moving images are displayed. afterimages cause a big problem.